User's Manual

User’s Manual of LRP-822CS/LRP-1622CS
116
erted after the MAC source address, but before the original Ether Type/Length or Logical
Link Control. Because the packet is now a bit longer than it was originally, the Cyclic Redundancy Check (CRC) must be
recalculated.
Adding an IEEE802.1Q Tag
Data Old CRC
The Ether Type and VLAN ID are ins
Dest. Addr. Src. Addr. Length/E. type
Dest. Addr. Src. Addr. E. type Tag Length/E. type Data New CRC
Priority CFI VLAN ID
Original Ethernet
New Tagged Packet
■ Port VLAN ID
arrying the 802.1Q VID information) can be transmitted from one 802.1Q compliant network
twork devices (and indeed, the entire
itch has a PVID. 802.1Q ports are also assigned a PVID, for use within the switch. If no VLAN are
e
erent the switch will drop
can coexist on the same network.
can have only one PVID, but can have as many VID as the switch has memory in its VLAN table to store them.
Because some devices on a network may b ision at each port on a tag-aware device before
itte tag If the transmitting port is connected to a
ntagged. If the transmitting port is connected to a tag-aware device, the packet
should be tagged.
■ Default VLANs
T nitially configures one VLAN, VID efault." The factory lt setting ns all ports on the Switch to
w VLAN are configured in Port-based mode, their respective member ports are removed from the "default."
Packets that are tagged (are c
device to anothe
r with the VLAN information intact. This allows 802.1Q VLAN to span ne
network – if all network devices are 802.1Q compliant).
Every physical port on a sw
defined on the switch, all ports are then assigned to a default VLAN with a PVID equal to 1. Untagged packets are assigned th
PVID of the port on which they were received. Forwarding decisions are based upon this PVID, in so far as VLAN are
concerned. Tagged packets are forwarded according to the VID contained within the tag. Tagged packets are also assigned a
PVID, but the PVID is not used to make packet forwarding decisions, the VID is.
Tag-aware switches must keep a table to relate PVID within the switch to VID on the network. The switch will compare the VID
of a packet to be transmitted to the VID of the port that is to transmit the packet. If the two VID are diff
the packet. Because of the existence of the PVID for untagged packets and the VID for tagged packets, tag-aware and
tag-unaware network devices
A switch port
e tag-unaware, a dec must be made
packets are transmitted – should the packet to be transm d have a or not?
tag-unaware device, the packet should be u
he Switch i = 1, called "d d
efau assig
the "default". As ne